Add missing initialization found due to a valgrind false positive.

This field is never inspected in the object state initialized by this
constructor; however, initializing it seems reasonable, since it has
a meaningful value.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213499 91177308-0d34-0410-b5e6-96231b3b80d8